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NIKA2+ hardware adaptation to focuses differences

Historical context

In the first runs of NIKA2 in 2015 ref, we noticed that the 2mm beam was severely degraded and concluded that the dichroic #1 was severely distorted. After years using the dichroic #2, which was flat but had a problem of misplaced transition in the 1 mm band, and an unsuccessful try with the dichroic #3 which had a distortion problem similar to dichroic #1 ref, we finally came back to dichroic #1 for the NIKA2+ upgrade in March 2025 ref. This time the dichroic #1 was mounted differently than in the early runs 10 years before and than the dichroic #3 in 2017. The choice of mounting the dichroic #1 membrane in sandwich between 2 aluminum rings maintained in the copper mount with a copper flange screwed on the mount with a torque of 0.5 N*m, result from a series of tests conducted in IRAM lab these last years [[ref], which goal was to understand the cause of the distortion and find a way to suppress it or at least minimize it down to an acceptable value.
From the lab tests we were expecting a residual distortion in NIKA2 at cold temperature between 0 and 1 mm amplitude. The first order effect of the distortion is a displacement of the image surface in the cryostat, that can be compensated thanks to a displacement of the KID array. Beyond 1 mm distortion some second order aberration effect start to be more and more significant and degrade the beam even at the best focus.
A collection of copper rings between 1 and 10 mm thickness was fabricated, to be used as possible shims to be placed between the KID array holder and its mount on the cryostat, in order to shift the array position along the optical axis.

For the March 2025 intervention we changed a number of elements in the NIKA2 cryostat ref, but decided to not change the position of the arrays since we didn't knew exactly what would be residual distortion of dichroic #1 with its new mount in the NIKA2 cryostat environment.
During one year of using NIKA2 on sky, between run ref and run ref, we monitored the focus differences between the arrays. All these values are gathered in a document showing the evolution of the focus values and differences in a few plots ref. We noticed a constant focus offset between Array 1 and Array 3 (possibly due to a small residual distortion of the polarizer or a small difference in optical path length), and changing focus offset between Array 2 and the other arrays, passing from a small value with consequence on the beam to an unacceptable value where the 2 mm beam quality is very severely affected when the focus is set for the 1 mm Arrays. Somme Zemax simulations allows visualizing this effect with plots quantifying this effect ref. After each full warmup of the cryostat, the focus difference between Ar2 and the others at the following cool down was a bit more degraded, up to reaching an apparent asymptote at the last warmup in January 2026.

The residual distortion of the dichroic, its evolution with warmups and the possible measures to mitigate is were discussed at several NIKA2 meetings, in particular the following regular meetings ref, and the following dedicated one ref. In these meetings were discussed also another non optimal element in NIKA2+, the new Array 3, which is behaving better than the old one but performing significantly less good as the new Array 1, so that its contribution to the overall 1mm channel is weak. The discussions in the dedicated meeting leaded to create the following table of possible actions ref. From all these discussions we ended up with the following plan for this intervention in March 2026:

• Change the current Array 3 installed since March 2025, with a another one, which is a twin of the current Array 1, fabricated at the same time (2018) with the same parameters, but for which 1 of the 8 multiplexed lines of KID has stopped working during a lab test a few years ago. Connect this faulty line to box-S, which has always shown noisier signal than the others, probably due to a LNA not performing as well as the others.
• Insert 10 mm thick shim in the Array 2 mount to compensate for the 1.2 mm focus difference with Array 3 caused by the distorted dichroic, assuming the distortion is downward, like in all the lab experiments.
• Insert 2 or 3 mm thick shim in the Array 1 mount to compensate for the 0.25 mm focus difference with Array 3.

Actual plan for the intervention starting on March 10th, 2026

One week before the intervention, comparing focus values over the last year with typical EMIR focus values, Samuel came to the conclusion that the dichroic distortion must be upward and not downward as we thought. He came to this conclusion because in the optical design of NIKA2 the optimal focus for NIKA2 is obtained by increasing the distance between M1 and M2 by 1 mm compared to the optimal focus for EMIR. The value to enter in Zemax for NIKA2 to displace M2 in that direction is -1 mm, while in PaKo this is +1 mm. Several colleagues from IRAM Granada looked for information attesting with 100% certainty that a positive value entered in PaKo correspond to an increase of the distance M1-M2. They found some clues, like this drawing found by Miguel ref, but no document clearing totally the uncertainty.
A distortion upward of the dichroic shortens the optical path on the 2mm band, so that we should not introduce 10 mm thick shims to compensate to focus shift, but reduce the mount length by 10 mm, which is not possible. An alternative is to make the focus on Array 2 with M2, and insert shims on Ar1 and Ar3 to compensate for the corresponding focus shift. But as Zemax simulations show ref, this has consequences on the 1 mm beam quality. Hopefully, this affect the beam only by a few %, so that the loss is small enough to be acceptable. As a consequence the plan changed to the following actions:

• Measure with a laser the displacement of M2 when a command is entered to change the focus Z.
• Change current Ar3 with the twin of the current Ar1, connecting its missing line to box-S.
• Insert 10 mm thick shim in the Ar3 mount.
• Insert 2 or 3 mm thick shim in the Ar1 mount.

Preliminary work before March 10th, 2026, and staff for the intervention

• NIKA2 warmed-up.
• Array 3 spare prepared.
• Spare leak detector shipped from IRAM Grenoble to IRAM Granada.
• Shims at the telescope.
• Tool box for opening NIKA2 cryostat.
• Torque wrench at the telescope.

Intervention team: Alessandro Monfardini, Martino Calvo, Emile Prele (PhD student at Neel), David John, Samuel Leclercq

AM, MC, EP, DJ at the telescope on Monday, March 9th to prepare everything for the intervention on Tuesday morning.

March 10th, Tuesday

Measuring displacement of M2 with a laser, when changing focus Z value with OHB control system, and with PaKo:

Laser in M1 vertex distance to M2 before any focus command: 13.061 m. - few mm in OHB control soft => 13.058 m ; 0 mm => 13.060 m ; +10 mm => 10.071 m ; -8 mm in PaKo => 13.052 m.
==> As expected, for both the OHB control software and in PaKo a positive value increases the MA-M2 distance. This means the dichroic distortion is upward, as feared.

Opening NIKA2:

First SURPRISE: the dichroic is much more distorted than we expected, the distortion is visible with the naked eye !! As expected it is upward. We measure it:

==> Distortion height = distance from a flat reference to the dichroic edge perpendicular to its surface - distance from a flat reference to the dichroic center perpendicular to its surface = 16-9 = 7 mm !!
This is so spectacular (but logic somehow that it's bigger at warm temperature than the 1.17 mm at cold temperature deduced from focus differences), that everyone here wonder whether we should really stick to the plan or rather reconsider it and try to do something about this distortion, namely dismounting the dichroic and reset its mechanical constraints to let it retrieve a flat shape. We decide to push a bit the Zemax simulations to weight various scenarios, and inform the persons involved in emails discussions since the discovery that the distortion should be upward and not downward the week preceding the intervention (NP, MS, SB, CK, AS, AB, MCF, IR, JT, JMP).

Other important observation looking at the Ar2 mount: it's possible to reduce its thickness by 2 mm, if we dismount it and mill it in the telescope machine shop. In terms of beam quality, its preferable to do that rather than introducing thicker shims on the 1 mm arrays. We decide to go for it, Victor takes care of the milling.

... result of email exchanges ... option 1, option 2 ... KS ... go for option 2

Dismounting dichroic to rest its distortion to 0. Second SURPRISE: after releasing the rings, the dichroic membrane does not recover a flat shape as it did in the lab !!

==> Distortion height = distance from a flat reference to the dichroic edge perpendicular to its surface - distance from a flat reference to the dichroic center perpendicular to its surface = 16-13 = 3 mm! The membrane is not flat, but at least the distortion reduced by a factor 2.
After discussions on the various options offered to us we decide to fix again the membrane in its mount, applying a torque of 0.15 N*m torque on the screws of the copper flange instead of 0.5 N*m previously (the hope is that this gives more room to the dichroic to expand during warmup before being blocked again by the pressure of the rings, and thus reduce or even cancel the hysteresis effect that we suspect at each thermal cycling of the cryostat). ... We decide to introduce a 3mm thick shims on Ar3, and not touch Ar1. So the total displacements between Ar3 and Ar2 is +3-(-2) = 5 mm. This is half the thickness of the thickest shim from the original plan. So if we managed to reduce the dichroic distortion by a factor 2, we should be right on spot in terms of focus equality between all arrays.

We mount the new array 3 with its 3mm shim, re-mount array 2 with its 2 mm thinned mount, and measure again the dichroic distortion after mounting it back in NIKA2.

End of Day 1.

March 11th, Wednesday

Closing NIKA2.